Devices such as a cellular phone, a digital camera, a PDA, a touch panel display, a large-screen television, and contact-less power transfer show a tendency of further prevalence.
A tempered glass, which is produced by applying tempering treatment to glass through ion exchange treatment or the like, is used for those applications (see Patent Literature 1 and Non Patent Literature 1).
In addition, in recent years, the tempered glass has been more and more frequently used in exterior parts of, for example, digital signage, mice, and smartphones.
A related-art device includes a display module, a touch panel sensor, and the tempered glass (protective member). In recent years, a method involving forming the touch panel sensor on the tempered glass has started to be adopted in order to achieve a reduction in weight or a reduction in thickness. As a result, such protective member is required to: (1) have high mechanical strength; (2) have high flaw resistance; (3) be less costly; (4) have low density; (5) have sufficiently high acid resistance for preventing denaturation of its surface in acid treatment at the time of formation of the touch panel sensor; and (6) be free of a substance having a high environmental load.